ACL2 keeps track of the commands that you have executed that have
extended the logic or the rule data base, as by the definition of
macros, functions, etc. Using the facilities in this section you
can review the sequence of commands executed so far. For example,
you can ask to see the most recently executed command, or the
command10 before that, or the command that introduced a given
function symbol. You can also undo back through some previous
command, restoring the logical world to what it was before the given
command.

The annotations printed in the margin in response to some of these
commands (such as `P', `L', and `V') are explained in the
documentation for :pc.